Third-party PDF viewers are available, if you prefer not to use the Firefox built-in viewer. To switch from the built-in PDF viewer to another viewer:

Click the menu button and choose Options.Preferences.

Select the Applications panel.In the General panel, go down to the Applications section.

Find Portable Document Format (PDF) in the list and click on it to select it.

Click on the drop-down arrow in the Action column for the above entry and select the PDF viewer you wish to use.

Note: If you want to view PDF files in Firefox using a plugin, instead of in an external PDF reader application, select Use PDF plugin name (in Firefox). For more information, see View PDF files in Firefox.

Close the about:preferences page. Any changes you've made will automatically be saved.